In U.S. Pat. No. 3,842,828 there is disclosed a ventilator which utilizes an intermittent mandatory ventilation (hereinafter IMV) concept which has been found to be very successful when applied to babies. However, in the IMV concept utilized in that ventilator, a constant flow is provided into the breathing circuit. In order to accommodate adults with a similar concept, much larger flows are required which, in view of the high cost of gases, would be almost prohibitive to utilize. There is, therefore a need for a new and improved ventilator having IMV capabilities which can be utilized in conjunction with adults as well as infants.